This invention relates to improvements in fluid cylinder assemblies having an elongate cylinder shell and movable plunger member therein of either the piston or displacement type. More specifically, the improvement relates to means for minimizing the outside diameter of the cylinder shell without adversely affecting the performance of the cylinder assembly.
The design of fluid cylinder assemblies is determined primarily by certain basic performance requirements such as required load capacity, required length of stroke and available pressure of the fluid supply for the cylinder assembly. For example, required load capacity and available pressure determine the minimum effective diameter of the plunger member (which can be of either the piston or displacement type), and required stroke determines the minimum interior length of the cylinder shell. Although it would be ideal if the minimum outside diameter of the cylinder shell could likewise be determined by such basic performance requirements, it has unfortunately been necessary to overdesign the cylinder shell beyond the outside diameter which would be needed merely to satisfy the foregoing basic requirements. Such overdesign adds substantially to the cost and space requirement of the cylinder assembly, which is highly disadvantageous in high-quantity applications of fluid cylinders and in those cases where the available space with respect to width or length of the cylinder is limited, such as in the manufacture of lift truck masts where visibility requirements and hoist chain space requirements limit the available space for hydraulic lift cylinders.
The reason why the outside diameter of the cylinder shell has required such overdesign is related to the fact that the cylinder assembly has an annular retainer fastened to the end of the cylinder shell from which the plunger member extends, such retainer functioning primarily to limit the extensibility of the plunger member by forcibly stopping it and thereby interfering with its hyperextension. The annular retainer surrounds the plunger member forming an inwardly-protruding radial lip near the end of the shell. The plunger member has a surface (which may constitute either the rod side of a piston or an outwardly-protruding annular lip adjacent the base of a displacement-type plunger) which interferes with and cannot pass the inwardly-protruding retainer lip when the plunger member reaches full extension, thereby preventing further extension. This interference imposes a great force upon the annular retainer in the direction of extension of the plunger member, equal substantially to the force exerted by the pressurized fluid within the cylinder upon the plunger member itself. The retainer resists this force by means of its fastened connection with the end of the cylinder shell, which normally is a threaded connection.
In the past, some such retainers have been in the form of caps which fit over the end of the cylinder shell and threadably engage the exterior surface thereof. However such externally-threaded retainer structures require a substantial thickness of retainer metal protruding annularly around the exterior or peripheral surface of the end of the shell to provide necessary thread strength. This increases the effective outside diameter of the cylinder assembly significantly beyond the outside diameter of the shell itself, causing severe space problems in certain applications as mentioned above.
To reduce this space problem, many modern cylinder designs feature threads formed on the interior surface of the cylinder shell, with the annular retainer threaded internally into the shell as shown, for example, in U.S. Pat. Nos. 2,438,285, 2,517,153, 2,783,744, and 3,136,221. These internally-threaded retainer structures are now commonly used in applications where space limitations dictate minimal outside diameter of the cylinder assembly. Such internally-threaded structures as shown in the first three of the aforementioned patents, however, introduce certain stresses into the cylinder shell which are of such magnitude as to increase the required minimum thickness of the cylinder shell wall to a value significantly greater than that which would be dictated by the aforementioned basic performance requirements of the cylinder assembly. Moreover, for reasons to be explained hereafter, the minimum space which must be required between the plunger member and the interior surface of the cylinder shell in all of the aforementioned patented structures is increased by the sealing needs of the internally-threaded structure for holding the annular retainer.
The reason why the internally-threaded annular retainer has a maximizing effect on these design factors is that the high degree of longitudinal force exerted through the retainer upon the internal shell threads by the plunger member at full extension imposes a severe outward mechanical moment on the shell wall material tending to radially spread or expand the end of the shell, resulting in a phenomenon known in the industry as "bellmouthing" of the end of the shell. Excessive bellmouthing can loosen the threaded connection between the retainer and shell to the extent that the retainer can be forced out of the end of the shell under sufficient force. Accordingly, in order to resist such bellmouthing of the shell end, the shell wall either must be made significantly thicker than would be required merely to resist tensile stresses resulting from fluid pressure within the shell, or its end must be enclosed by an annular groove in the retainer as shown in the aforementioned Walker U.S. Pat. No. 3,136,221.
Even when a grooved retainer structure is used as shown in U.S. Pat. No. 3,136,221, however, the bellmouthing phenomenon still dictates the minimum distance required between the plunger member and the interior wall of the shell, thereby also influencing the outside diameter of the shell, because of the fact that a compressible, resilient fluid seal must be located between the retainer and the cylinder shell to prevent leakage through the threads. (The absence of such a seal, as in the aforementioned U.S. Pat. Nos. 2,438,285 and 3,136,221, is not acceptable for many applications such as lift trucks because of external leakage of fluid through the threads and resultant fluid losses and deposits on warehouse floors.). Compression of such a resilient seal is required to prevent leakage; moreover there can be no substantial clearance between the retainer and the shell wall in the area of the seal or the seal will gradually extrude into the clearance and thus deteriorate. If the seal were located near the end of the shell, where its presence would minimize the space required between the plunger and the interior surface of the shell, any tendency of the shell toward bellmouthing would both relieve the needed compression on the seal and permit the aforementioned harmful clearance between the shell wall and the retainer in the area of the seal. Unfortunately the tendency toward bellmouthing occurs to some degree in all previous cylinder assemblies, even those wherein the end of the shell wall is enclosed in a grooved retainer of the type shown in the aforementioned U.S. Pat. No. 3,136,221 because such a vertically-walled groove requires adequate clearance from the shell wall which it encloses to permit initial installation of the retainer. Such clearance permits sufficient bellmouthing to adversely affect the compression on any seal located near the end of the shell, and also provides a space into which the seal could eventually extrude thereby damaging it.
Accordingly, rather than being located near the end of the shell at a position exterior of the retainer threads where there would be ample room for it, the seal must instead be located more deeply within the shell at a position interior of the threads as shown, for example, in the aforementioned U.S. Pat. Nos. 2,517,153 and 2,783,744. The structure for mounting the seal at such depth requires more space than would otherwise be needed between the plunger member and the interior surface of the cylinder shell, thereby adding to the outside diameter of the entire assembly.
The aforementioned deep seal location has a further magnifying effect on the outside diameter and cost of displacement-type cylinder assemblies since, in order to install the seal within the shell at a greater depth than the internally-formed threads, such threads must be relieved (i.e. recessed) in a radially-outward direction from the deeper interior surface of the shell so that the seal may slide by the threads during installation without contacting them and being damaged by them. This relieving of the threads requires either thicker shell wall material to provide sufficient thread strength, or else a preformed outward deformation or flaring of the shell wall in the area of the threads to provide the needed relief. Making the shell wall thicker to provide such relief adds both to the cost of materal and outside diameter, while preformed outward flaring of the shell requires an annealing step which adds cost and also adds to the effective outside diameter of the shell. While such relief would always be necessary, regardless of the depth of the retainer seal location, in piston-type cylinders because of the additional requirement of passing the piston seal by the threads during initial installation of the piston, such relief could be eliminated in displacement-type cylinder assemblies having no piston seal were it not for the deep location of the retainer seal.